Emergency collective actuator and method for a helicopter

ABSTRACT

A helicopter includes a rotor system having a rotor with an adjustable pitch that is controlled at least in part by a pilot using a collective control and which helicopter generates a Low RPM signal that is indicative of a threshold low rotational speed of the rotor. An actuator arrangement can move the collective control by exerting a force on the collective control such that the pilot is able to overcome the actuator force but which otherwise can move the collective control from a current operational position toward a minimum pitch position. A clutch can co-rotate with a motor to serve in transferring the actuation force to reduce the adjustable pitch and can slip relative to the actuator shaft assembly responsive to an application of a counterforce applied by the pilot to the collective control such that the counterforce overcomes the actuation force.

RELATED APPLICATIONS

The present application is a continuation application of copending U.S.patent application Ser. No. 13/479,130 filed on May 23, 2012, which is acontinuation-in-part of U.S. patent application Ser. No. 12/953,294filed on Nov. 23, 2010 and issued as U.S. Pat. No. 8,360,369 on Jan. 29,2013, which itself claims priority from U.S. Provisional ApplicationSer. No. 61/264,181 filed on Nov. 24, 2009, each of which applicationsare hereby incorporated by reference in their entireties.

BACKGROUND

The present invention is related at least generally to helicoptercontrol systems and, more particularly, to an emergency collectiveactuator and associated method for a helicopter.

It is recognized in the prior art such as is exemplified by U.S. Pat.No. 4,667,909 (hereinafter the '909 patent) that a sudden power failureduring the flight of a helicopter requires the immediate attention ofthe pilot to convert to autorotation by lowering the collective pitch ofthe main rotor blades of the helicopter. A failure to timely reduce thecollective can result in stalling the rotor blades. Such stalling of therotor blades will generally produce a catastrophic crash wherein thehelicopter, quite literally, falls from the sky. One example of such anaccident, which likely involved a rotor stall, occurred in the UnitedKingdom in March of 1998 and is the subject of AAIB Bulletin no. 11/98.Such an accident will generally be fatal to anyone onboard the aircraft.The particular helicopter that was involved in this accident was theRobinson R22, which is a lightweight helicopter having a low-inertiarotor system. It should be appreciated that a low-inertia rotor systemcan be stalled more easily than a rotor system having a greater level ofinertia. The subject accident report outlines operational conditions forthe Robinson R22 under which rotor speed will decay to an unrecoverablevalue in less than 1 second during a climb.

The prevalent teaching in the prior art with regard to avoiding rotorstall appears to be to simply instruct the pilot to lower the collectivesetting of the rotor immediately in the event of an engine failure topreserve inertia in the rotor system. In practice, Applicants believethat it is questionable how effective this advice might be relative tolow rotor inertia helicopters since engine failure appears to berelatively uncommon. Hence, it is difficult for the pilot to immediatelyreact to a situation that has never been fully experienced firsthand.Even during training, Applicants believe that few student pilots areprovided with actual experience either in simulation or actual flightthat would realistically duplicate an actual engine failure. The lack ofsuch training is attributed to a certain enhanced level of danger thataccompanies the training itself, since full down auto-rotation landingsrequire considerable skill in low rotor inertia helicopters and mightresult in damage to the helicopter. In this regard, flight instructorsare advised to warn a student pilot prior to initiating trainingexercises relating to power failure simulation, at least in the RobinsonR22.

The '909 patent appears to be consistent with the prior art inrecommending that the pilot should react immediately and seeks toalleviate the problem by relocating the collective control. Applicantsbelieve that this approach is of limited value since the collectivecontrol is traditionally located by the pilot's left hand. It isbelieved that most experienced pilots would object to relocating thiscritically important control, since reaction time could at leastarguably be increased simply by moving the collective control to anon-traditional location.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

In general, an apparatus and method are described for use with ahelicopter including a rotor system having a rotor with an adjustablepitch that is controlled at least in part by a pilot using a collectivecontrol and which helicopter generates a Low RPM signal that isindicative of a threshold low rotational speed of the rotor. In oneaspect, an actuator arrangement is configured for moving the collectivecontrol by exerting a force on the collective control such that thepilot is able to overcome the force but which otherwise moves thecollective control from a current operational position toward a minimumpitch position. A control arrangement is configured for receiving theLow RPM signal and for responding to the Low RPM signal by activatingthe actuator arrangement for at least a predetermined period of time toapply the force to move the collective control from the currentoperational position to the minimum pitch position in an absence of acollective control input from the pilot. In one feature, the controlarrangement is further configured for entering a lockout intervalimmediately following the predetermined period of time, during whichlockout interval the Low RPM signal is disabled from activating theactuator arrangement.

In another aspect, the Low RPM signal is received and responded to byexerting a force to move the collective control from the currentoperational position to the minimum pitch position for a predeterminedperiod of time in an absence of a collective control input from thepilot such that the pilot is able to overcome the force but whichotherwise moves the collective control from a current operationalposition toward a minimum pitch position. In one feature, a lockoutinterval is entered immediately following the predetermined period oftime during which lockout interval the Low RPM signal is disabled fromcausing the collective to move.

In still another aspect, an apparatus and associated method aredescribed for use with a helicopter including a rotor system having amain rotor with an adjustable pitch that is controlled at least in partby a pilot using a collective control. The apparatus includes anactuator arrangement that is configured to change the adjustable pitchby exerting an actuation force such that the pilot is able to overcomethe actuation force using the collective control but which otherwisebiases the adjustable pitch into a minimum collective pitch position. Acontrol arrangement is configured to receive a signal that is indicativeof a low rotor RPM condition and to respond to the signal by activatingthe actuator arrangement to exert the actuation force.

In an embodiment, a motor includes a motor shaft and the motor shaftsupports an actuator shaft assembly which includes a clutch disk thatco-rotates with the motor shaft. A clutch assembly can be supported bythe clutch disk and can be configured to co-rotate with the actuatorshaft assembly in the given direction to serve in transferring theactuation force to reduce the adjustable pitch and to slip relative tothe actuator shaft assembly responsive to an application of acounterforce applied by the pilot to the collective control such thatthe counterforce overcomes the actuation force to thereby at leastmaintain a current setting of the adjustable pitch.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be illustrative rather than limiting.

FIG. 1 is a diagrammatic view, in elevation, of a helicopter collectivecontrol coupled with an emergency collective actuator of the presentdisclosure.

FIG. 2 is a diagrammatic view, in elevation and partially inperspective, showing one embodiment of an emergency collective control.

FIG. 3 is a block diagram showing one embodiment of a control sectionwhich can form part of the emergency collective control.

FIG. 4 is a timing diagram which illustrates timer output controlsignals and a motor drive signal produced by the control section.

FIG. 5 illustrates one embodiment of a method for operating theemergency collective actuator of the present disclosure.

FIGS. 6-9 are diagrammatic views, in elevation, showing details withrespect to another embodiment of actuation components of the emergencycollective actuator.

FIG. 10 is a diagrammatic, partially cut-away end view of anotherembodiment of actuation components of the emergency collective actuator.

FIG. 11 is a diagrammatic view, in elevation, showing another embodimentof the emergency collective actuator showing the collective control in araised position.

FIG. 12 is a diagrammatic plan view, in partial cross-section, showingfurther details of the embodiment of the emergency collective actuatorof FIG. 11.

FIG. 13 is a diagrammatic view, in elevation, showing the embodiment ofthe emergency collective actuator of FIG. 11, but with the emergencycollective actuator in an engaged mode having lowered the collectivecontrol.

FIG. 14 is a partially cut-away diagrammatic view, in elevation, of theemergency collective actuator as shown in FIG. 13, but showing thecondition of the mechanism when the pilot counteracts an actuation bythe emergency collective actuator.

FIG. 15 is a block diagram showing another embodiment of a controlsection which can form part of the emergency collective actuator of thepresent disclosure.

FIG. 16 is a diagrammatic view, in elevation, showing a helicopter whichincludes an embodiment of the emergency collective actuator of thepresent disclosure in which at least ground proximity detection is usedfor purposes of controlling the emergency collective actuator of thepresent disclosure.

FIG. 17 is an installed diagrammatic perspective view of anotherembodiment of the emergency collective actuator of the presentdisclosure, shown here with a collective control of the helicopter in afully lowered position.

FIG. 18 is a diagrammatic perspective view of the embodiment of theemergency collective actuator of FIG. 17 shown in a further enlargedview in isolation from the helicopter, but corresponding to the fullylowered position of the collective.

FIG. 19 is another installed diagrammatic perspective view of theembodiment of the emergency collective actuator of FIG. 17, but shownhere with the collective control of the helicopter in a fully raisedposition.

FIG. 20 is a diagrammatic perspective view of the embodiment of theemergency collective actuator of FIG. 19 shown in a further enlargedview in isolation from the helicopter, but corresponding to the fullyraised position of the collective.

FIG. 21 is a diagrammatic exploded view, in perspective, of theembodiment of the emergency collective actuator of FIGS. 17-20.

FIG. 22 is a diagrammatic perspective view which illustrates furtherdetails with respect to an embodiment of a cable drum that forms aportion of the embodiment of the emergency collective actuator.

FIG. 23 is another diagrammatic perspective view which illustratesfurther details with respect to the embodiment of the cable drum of FIG.22.

FIGS. 24 a-24 d are diagrammatic end views that illustrate therelationship between various components of the emergency collectiveactuator embodiment of FIGS. 19-23 in various operational states thatare of interest.

FIG. 25 is a diagrammatic view, in partial perspective, of anotherembodiment of the emergency collective actuator of the presentdisclosure.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments will be readily apparent to those skilled inthe art and the generic principles taught herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown, but is to be accorded the widest scopeconsistent with the principles and features described herein includingmodifications and equivalents, as defined within the scope of theappended claims. It is noted that the drawings are not to scale and arediagrammatic in nature in a way that is thought to best illustratefeatures of interest. Descriptive terminology such as, for example,upper/lower, uppermost/lowermost, vertically/horizontally and the likemay be adopted for purposes of enhancing the reader's understanding,with respect to the various views provided in the figures, and is in noway intended as being limiting.

Attention is now directed to the figures wherein like reference numbersmay refer to like items throughout the various views. FIG. 1 is adiagrammatic view, in elevation, of a helicopter collective control thatis generally indicated by the reference number 10 and which isrepresentative of the collective control in a helicopter such as, forexample, the Robinson R22. Collective control 10 is positioned on apanel 12 of the helicopter and is attached to a pivot 14 at one end. Ahandle end 20 of the collective control is configured for gripping by apilot and includes a throttle control that is actuatable by twistinghandle 20 as indicated by a double headed arrow 22. The pilot canincrease the collective by moving handle 20 pivotally in the directionindicated by an arrow 24 or reduce collective by moving handle 20oppositely.

Still referring to FIG. 1, an emergency collective actuator arrangementis generally indicated by the reference number 100. The emergencycollective actuator arrangement includes a main unit 110 that isconfigured to receive a Low RPM signal that is generated by thehelicopter. In the instance of the Robinson R22, the Low RPM signal isproduced when the rotor speed falls to 97% of normal or less. Normally,the Low RPM signal is used to actuate a warning horn and light to drawthe pilot's attention to the status of the rotor speed. The manner inwhich main unit 110 uses the Low RPM signal will become evident in thediscussions which follow.

A control cable 112 extends from main unit 110 to a clamp arrangement114 that is attached to an intermediate position 116 on collectivecontrol 10. Any suitable control cable and clamp arrangement can be usedso long as the attachment is reliable. The clamp arrangement should belocated sufficiently away from the pivot end and the handle end of thecollective control so as to avoid any interference with normal operationor with actuation by the pilot. In response to the Low RPM signal, mainunit 110 retracts cable 112 so as to lower the collective. Thisoperation proceeds automatically in response to reception of the Low RPMsignal and can be initiated essentially instantaneously, at least from apractical standpoint, e.g. in 0.001 seconds (1 ms) or less in responseto the Low RPM signal, but in any event significantly less than thereaction time of even an attentive pilot. In one embodiment, thereaction time of main unit 110 can be adjustable, and yet remain farless than the typical reaction time of the pilot to provide asignificant safety enhancement. Typically, however, the circuitry willbe allowed to react as quickly as it is capable of reacting withoutintroducing any additional delay. In this regard, under certain flightconditions outlined in the AAIB accident report that is discussed in theBackground Section, Applicants believe that the required reaction timemay be so short as to present a virtually unrecoverable condition in theabsence of the use of emergency collective control 110 and itsassociated method. As will be described in further detail, theretraction force that is applied to collective control 20 by emergencycollective actuator 100 can be overcome by the pilot, although the forceis sufficient to inform the pilot that the unit is attempting to lowerthe collective when the left hand of the pilot is holding collectivehandle 20. In this regard, the collective control may have a frictionsetting that can be engaged by the pilot to introduce additionalfriction at pivot 14 so that the pilot is able to at least momentarilyrelease his or her hand from the collective. The retraction force istherefore configured such that the collective can be lowered by theemergency collective control unit even if the highest setting of thefriction control is in use.

Referring now to FIG. 2, a diagrammatic illustration, partially in aperspective view and partially in an elevational view is providedshowing one embodiment of emergency collective control 100. In thisembodiment, as part of main unit 110, a motor 200 includes an outputshaft 202 that rotates a drum 204. The latter can be received directlyon output shaft 202, for example, at each one of a pair of opposing endwalls 210, only one of which is visible. Drum 204 defines a spiralgroove 212 that can include one or more turns around the periphery ofthe drum. Groove 212 can have a suitable profile in cross-section suchas, for example, a V-shape or a U-shape. Cable 112 can be receivedaround the drum to form at least one turn therearound. The drum may beformed from any suitable material such as, for example, a lightweightaluminum alloy. One end 214 of cable 112 extends to and is attached to afirst end 218 of a spring 220. In the present embodiment, the spring isa helical coil spring, although any suitable type of spring may be usedso long as the spring is able to maintain some degree of tension on thecable irrespective of the position of collective control 20. In thisregard, the spring serves to maintain tension and accommodate movementof the collective without providing any noticeable resistance toactuations by the pilot.

Cable 112 may be attached to the spring in any suitable manner such as,for example, by using a crimping clamp 222. An opposing end 224 of thespring is suitably fixedly attached to a convenient location on thehelicopter such as, for example, the bottom of panel 12 (showndiagrammatically) such that resilient tension is continuously applied tocable 112. An opposing end 230 of cable 112 extends for attachment tointermediate position 116 of collective control 20. One arrangement forattaching cable 112 to the collective control is shown in an enlargedview within a dashed circle 240. A clamping ring 250, typically a metalband, is shaped to fit around the periphery of the collective controland is tightened about the periphery using a suitable fastener 252 suchas, for example, a rivet. A cable end fitting 254 can be attached to end230 of the cable, for example, by crimping/compressing and can have abifurcated shape with two opposing tines 258 (only one of which isvisible), each of which defines an opening 260. A pin, which is notshown due to illustrative constraints, is receivable in openings 260 andthrough ears 262 of clamping ring 250 such that end fitting 254 canpivot about the pin.

In this embodiment, motor 200 is configured to rotate with the drum inthe event that the pilot pulls the collective upward to overcome theretraction force that is provided from motor 200. The number of turns ofcable 112 around the drum in spiral groove 212 should provide sufficientfrictional engagement between the drum and cable so as to avoid slippingof the cable relative to the drum in view of tension that is provided byspring 220. An electronic control section forms another part of mainunit 110 and is used to provide electrical drive to motor 200 with powerbeing provided from the helicopter on a line 302 and Low RPM signalbeing provided on a line 304, as will be further described immediatelyhereinafter.

Turning to FIG. 3, electronic control section 300 is shown in blockdiagram form. As noted above, operational power is received from thehelicopter on line 302 (shown in FIG. 2) and the Low RPM signal isreceived on line 304. Power is provided to each of a timer 310, an ANDgate 312 and a driver 314. It is to be understood that a power supplysection can be provided in the event that these components have varyingpower requirements with respect to one another and/or with respect tothe power that is available directly from the helicopter. For purposesof descriptive convenience, it will be assumed that control section 300employs active-high logic, although active-low logic can just as readilybe used. Initially, the Low RPM signal is provided as a logic highsignal to one input 316 of AND gate 312 and to timer 310. It isconsidered that one having ordinary skill in the art may readilyimplement appropriate circuitry in a wide variety of forms with thisoverall disclosure in hand.

Having described FIGS. 1-3 in detail above, attention will now bedirected to operational details of one embodiment of emergencycollective actuator 100 with additional reference to the timing diagramof FIG. 4. This timing diagram illustrates the Low RPM signal in a plot400 showing the signal versus time. At a time t₁, the Low RPM signaltransitions to a high, alert status at which time the helicopter hornwould sound and the Low RPM indicator light illuminates. It should beappreciated that various conditions may be encountered during a givenautorotation based, at least in part, on the pilot's choice of asuitable landing site. For example, in order to achieve maximum range inthe Robinson R22, a forward speed of 70 knots is recommended with 90% ofnormal rotor rotational rate. Of course, under these conditions, the LowRPM horn will sound continuously. Under other conditions, however, thepilot may achieve more than 97% of normal rotor rotational rate afterthe initial sounding of the Low RPM horn such the horn is at leasttemporarily extinguished. One example of an event that would extinguishthe Low RPM signal would be for the pilot to initiate autorotation withaft cyclic which causes the main rotor speed to rise above 97% ofnormal. Another example of an event that may result in a change in thestatus of the Low RPM signal resides in what is typically referred to asa “flare” that is used to slow the speed of the helicopter duringautorotation immediately prior to landing. During this flare, main rotorspeed increases and then decreases as the pilot adds collective justbefore landing. Accordingly, it should be appreciated that the Low RPMsignal may toggle between active and inactive conditions duringautorotation depending on the changing rotational status of thehelicopter main rotor. In the present example, which is not intended asbeing limiting, the initial low RPM event ends at a time t₂.Subsequently, at t₃, the Low RPM signal again becomes active until t₄.

Timer 310 of FIG. 3 may operate according to a timer output plot 402 ofFIG. 4 which is provided to one input 404 of AND gate 312. In responseto the Low RPM signal, timer 310 produces a timer pulse 410 that ispresent at input 404 of AND gate 312. The timer pulse is indicated ashaving an overall duration of TP extending from time t₁ to t_(x). Theinitiation of the timer pulse interval can take place, for examplewithin 1 millisecond of the Low RPM signal based on any time delayintroduced by timer 310. From a practical standpoint, and in terms ofhuman perception, timer output 402 can become active at t₁. A motordrive signal 420 is generated at an output 422 of AND gate 312. So longas both inputs of AND gate 312 are active/high, output 422 of the ANDgate will also be active/high. If either one of the inputs of the ANDgate is low, however, its output will likewise be low. In the presentexample, Low RPM signal 400 and timer output 402 are both active for theduration TP of timer pulse 410 such that motor drive signal 420 includesa motor drive pulse 430 that is of a duration MD which corresponds toduration TP of the timer pulse, at least from a practical standpoint,although a time delay may be introduced by the circuitry that isimperceptible by human observation. The duration of timer pulse TP canbe customized based on a particular helicopter application, however, TPshould be long enough to allow for a relatively slow reaction time onthe part of the pilot. In this regard, Applicants note that it is oftendesirable to accommodate potential reaction times of several seconds onthe part of the pilot. For this reason, the predetermined interval oftimer pulse, TP, and therefore MD may be of six seconds or longerduration, although this is not a requirement; the predetermined intervalmay be any length, including a length with no end-point. For purposes ofdriving motor 200, motor drive signal 420 is provided to driver 314which provides current to the motor if the electrical currentrequirements of the motor cannot be satisfied directly by AND gate 312.

Continuing with a description of the operation of emergency collectiveactuator 100, it should be appreciated that a pulse 440 which is presentfrom t₃ to t₄ of Low RPM signal 400, in the present example, is notreflected by motor drive signal 420 for the reason that pulse 440 occurswell after t_(x) which terminates timer pulse TP. In this regard, afterissuing a timer pulse, timer 310 is configured to enter a LockoutInterval LI during which time the timer output is low such that anyactive signal events that might occur on the Low RPM signal cannotinfluence motor drive signal 420. The Lockout Interval can be of aduration that is sufficient to ensure enough time for a full autorotation to the ground such as, for example, 5 minutes or longer. Inthis way, emergency collective actuator 100 reacts to an initial low RPMevent immediately and advantageously need not counter any subsequentactuation of the collective control by the pilot which is requiredduring autorotation, for example, such as may occur during the flare, asdiscussed above. Overall circuitry delays can be managed to a degreethat causes the collective to actuate immediately in terms of humanperception responsive to the Low RPM signal, for example, the overalldelay to start motor rotation may be 1 millisecond or less, althoughlonger delays are also acceptable and still faster than the reactiontime of a typical pilot. It should be appreciated, however, that thepilot can counter any actuations by the emergency collective control atany time if he or she so chooses, even during motor drive interval MD.In this regard, for purposes of low-level hovering and maneuvers, thepilot will have his or her hand on the collective. At such sufficientlylow altitude, pilots are trained to increase the collective since it isnot practical to try to increase the amount of energy stored in therotor system using autorotation. In the event that the pilot feels anactuation that is attempting to lower the collective at such lowaltitude, the pilot can be trained to resist the actuation and increasethe collective.

Turning now to FIGS. 3 through 5, the latter illustrates one embodimentof a method for the operation of the emergency collective actuator,generally indicated by the reference number 500. The method starts at502 when the emergency collective actuator system is powered up. At 504,motor 200 is maintained in an OFF status. At 506, timer 310 (FIG. 3) isreset to zero and held ready for triggering. Step 508 then monitors theLow RPM signal and executes in a continuous loop so long as the Low RPMsignal is low/inactive. When the Low RPM signal becomes high/active,step 510 starts timer 310 (at time t₁ of timer waveform 402 in FIG. 4).The timer output remains high/active until t_(x), as shown in waveform402 which can be, for example, six seconds as described above. Step 512checks the timer signal following t₁. If the timer output ishigh/active, execution enters step 514 which reconfirms that the Low RPMsignal is active/high. If the Low RPM signal is high, step 516 turns onmotor 200 or maintains the motor in an ON status so as to retract thecollective. Operation then returns to step 512. If at step 514, the LowRPM signal is determined to be inactive, step 520 turns motor 200 offand execution returns to step 512. When step 512 determines that thetimer output is low, subsequent to t_(x) in timer plot 402, step 522turns motor 200 off. Operation then proceeds to step 526 which monitorstimer output 402 for the expiration of the Lockout Interval, shown as LIin timer output plot 402. One suitable value for the Lockout Interval is5 minutes, as described above. Following the Lockout Interval, operationreturns to step 504.

Turning now to FIG. 6, another embodiment is illustrated, in adiagrammatic perspective view, showing motor 200 which rotates a pulley600. One suitable motor has been found to be the MFA 942D series gearedmotor that is available from Como Drills of the United Kingdom. It isnoted that this motor is suitable for all of the embodiments describedherein at least for the reason that the application of external torqueto the output shaft can freely rotate the gear assembly of the motor andthe motor itself. Other geared and non-geared motors, however, may alsobe found to be suitable. The pulley may be configured in any suitablemanner such as, for example, defining a groove for receiving cable 112,as illustrated. The pulley is mounted on motor shaft 202 to extendthrough a helical coil spring 602. One end 604 of the helical coilspring is fixedly attached, for example, to motor 200 or other suitablestructure such as a bracket (not shown) that supports the motor. Itshould be appreciated that any suitable form of spring may be used. Forexample, a planar clock-spring may be used employing a spiral winding.The attachment of end 604 may be performed in any suitable manner, forexample by using a fastener 606. An opposing end of the spring, which isnot visible in the present view, is attached to pulley 600 in anysuitable manner such as, for example, by using a fastener. Cable 212 canbe attached to pulley 600, for example, using a swaged fitting 610 atthe end of the cable which receives a suitable fastener 612 that isfixedly received in the periphery of the pulley. It is noted that theillustrated position of pulley 600 corresponds to the full up positionof collective control 10 (FIG. 1). At this position, spring 602 ispre-tensioned so as to apply a force in a direction 620, indicated by anarrow, such that the pulley takes up slack in cable 112 if the pilotlowers the collective. In this regard, spring 602 applies sufficientforce to rotate the motor output shaft in taking up the slack in thecable. Pulley 600 is configured with a diameter such that less than onerotation of the pulley takes place from a full up position to a fullylowered position of the collective control so that there is no need forcable 112 to overlap on itself. In response to the Low RPM signal, motor200 rotates pulley 600 in the direction of arrow 620 so as to lower thecollective in accordance with the descriptions above. Motor 200 shouldhave sufficient torque to apply additional tension to spring 604 as themotor lowers the collective. Since less than one turn of the pulley isneeded, the added torque that is needed to further wind the spring isreadily manageable. In another embodiment, motor 200 may be suppliedwith a relatively small current which causes the motor to rotate pulley600 in the direction of arrow 610 so as to take up any slack in cable112 but without applying enough torque to lower the collective duringnormal flight conditions. In this later embodiment, spring 602 is notneeded.

FIG. 7 is a diagrammatic perspective view of another embodiment in whichcable 112 extends through the pulley groove beyond a capture band 630that is fixedly attached to the pulley periphery in any suitable mannersuch as, for example, by welding. Capture band 630 may be formed in anysuitable configuration so long as cable 112 is able to freely movevertically in the pulley groove. In instances where the cable diameteris greater than the width of the pulley groove, capture band 630 may bein a loop configuration so as to extend outward from the periphery ofthe pulley. A distal end of cable 112 supports an endpiece 632 that canbe attached to the cable end in any suitable manner such as, forexample, by swaging. The illustrated position of the pulley and captureband corresponds to a home or idle position under normal flightconditions such that cable 112 can move freely in the vertical directionresponsive to actuations of the collective by the pilot. The weight ofendpiece 632 can serve to prevent binding of cable 112 between captureband 630 and the pulley and generally maintain the orientation of thecable. As shown, cable 112 and endpiece 632 represent the fully raisedposition of the collective. The cable and endpiece are shown in phantomat the fully lowered position of the collective as indicated by thereference numbers 112′ and 632′, respectively.

Referring to FIG. 8 in conjunction with FIG. 7, motor 200 rotates pulley600 in the direction of arrow 620 responsive to the Low RPM signal. Atsome point in the rotation of pulley 600, endpiece 632 encounterscapture band 630. Since the endpiece is sized so as to be unable to passbetween capture band 630 and the pulley, cable 112 is retracted onto thepulley and the collective is lowered in a manner that is consistent withthe descriptions above. FIG. 8 illustrates the arrangement of thecomponents at the fully retracted or lowered position of the collective.The pilot may counteract the operation of motor 200 during theretraction period or following the retraction period by applying upwardforce to the collective which will counter-rotate the motor and pulley.

FIG. 9 is a diagrammatic end view, in elevation, of the embodiment ofFIGS. 7 and 8, further including a shield 650 that is arranged todeflect cable 112 and endpiece 632 such that interference withcomponents of the helicopter below the emergency collective actuator canbe avoided, if such interference is a possibility. Shield 650 may beformed from any suitable material such as, for example, metal and in anysuitable shape such as, for example, a trough shape. The shield may besupported in any suitable manner such as, for example, by using abracket that is attached to the helicopter.

Attention is now directed to FIG. 10 which is a diagrammatic partiallycut-away end view of another embodiment which resembles the embodimentof FIG. 6 with an exception that helical coil spring 602 is not used.Further, pulley 600 is shown as being partially cut-away to illustratethe presence of a micro-switch 680. The micro-switch can be of thenormally open type, although this is not a requirement, and includes anactuator 682 that extends through floor 684 of the pulley groove suchthat an open condition of the switch indicates that there is slack incable 112. On the other hand, when tension is applied to cable 112,pulley 600 rotates and applies tension to the cable such that the cablecauses the micro-switch to close. Indications of the status of themicro-switch can be provided on a pair of electrical leads 686, forexample, to control unit 300 (FIG. 2). Based on the status of themicro-switch, control unit 300 can monitor the switch to ensure thatthere is no slack in cable 112. Since the pulley rotates less than oneturn in its travel, the use of simple electrical connections isfacilitated such as, for example, a flexible wiring harness. Pulley 600may be machined to support micro-switch 680. The location of themicro-switch may be maintained in any suitable manner in the pulley, forexample, using an adhesive and/or one or more suitable fasteners.

Turning now to FIG. 11, another embodiment of emergency collectiveactuator arrangement 100 is diagrammatically illustrated. In thisembodiment, the actuator arrangement can be concealed below floorboardor deck 12 of the helicopter. In this regard, it should be appreciatedthat the deck can have a complex shape. Clamp 114 is pivotally connectedto an upper counterbalance arm 700 via a pin 702. A counterbalancespring arrangement 710 can be connected at an upper end to uppercounterbalance arm 700. A lower counterbalance arm 712 can be connectedat one end to a lower end of spring arrangement 710 and pivotallyconnected to a suitable fixed location on the helicopter at a lower endvia a pin 713. A helical coil spring 714 can be captured between upperand lower disks 716 a and 716 b using three shafts 718 (one of which isindicated). It is noted that the upper and lower counterbalance arms aswell as the counterbalance spring arrangement may be provided asoriginal equipment in a particular helicopter. In view of the presentexample, however, it is considered that one having ordinary skill in theart can readily implement an installation for a helicopter that isequipped with a different collective control configuration.

Referring to FIG. 12, in conjunction with FIG. 11, the former is adiagrammatic view, in partial cross-section, taken from a line 12-12that is shown in FIG. 11, and shown here to illustrate further detailswith respect to the components of emergency collective controlarrangement 100. First and second lever arms 720 a and 720 b arepivotally connected at a first end to a suitable fixed position in thehelicopter, for example, using a bracket 722 and pivot pin 724 such thatsecond ends 725 of the lever arms can rotate as indicated by a doubleheaded arrow 726. The lever arms are disposed at either side of uppercounter balance arm 700 and can pivot against a crown member 730 thatincludes an arcuate head which defines an aperture for receiving theshaft of the upper counter balance arm. Output shaft 202 of motor 200supports a disk 740 for selective rotation responsive to control section300. A lower actuator arm 742 is pivotally connected to disk 740 havinga pivot point 743 off-center with respect to the motor shaft. An upperend of lower actuator arm 742 is connected to an actuator springarrangement 746. In the present example, the actuator spring arrangementincludes a helical coil spring 748 having caps 750 a and 750 b mountedon its opposing ends with cap 750 b, in turn, attached to an upper endof the lower actuator arm. It should be appreciated that any suitabletype of spring arrangement can be used. An upper actuator arm 760 can bepivotally received between lever arms 720 a and 720 b at their secondends 725, for example, using a pin 762. As will be further described,rotation of motor 200 can apply a downward biasing force on collectivecontrol 10. Any suitable motor may be used such as, for example, a gearmotor.

Referring to FIG. 11, collective control 10 is shown in an uppermost,fully raised position. Disk 740 is oriented having pivot point 743 at anuppermost position such that actuator spring arrangement 746 does notapply a downward biasing force to the collective control. If the pilotmoves the collective downward, crown 730 moves downwardly away fromlever arms 720 a and 720 b, as counterbalance spring 714 is compressedby the pilot, such that the emergency collective actuator has no effecton the pilot's actuation. It is noted that the pilot can lower thecollective from any given position above minimum with no influence fromthe emergency collective actuator, since crown 730 moves downward andaway from lever arms 720 a and 720 b. The emergency collective actuatorarrangement may readily configured to overcome a collective frictionsetting that is intended to prevent inadvertent movement of thecollective, for example, if it is necessary for the pilot to move his orher hand away from the collective control.

FIG. 13 diagrammatically illustrates the collective control andemergency collective actuator after having fully lowered the collectiveas a result of motor 200 rotating disk 740 such that pivot point 743 isat a lowermost position. As disk 740 is rotated, which can take placeeither clockwise or counter clockwise, actuator spring arrangement 746pulls downward on ends 725 of lever arms 720 a and 720 b. The leverarms, in turn, engage crown 730 so as to compress counterbalance spring714 and thereby lower the collective. Motor 200 can rotate another 180degrees in either direction to release the collective, for example,after a predetermined time interval.

Referring to FIGS. 4, 11 and 13, control section 300 can be configuredin one embodiment with a microcontroller that is configured to generatea first pulse at t₁ having a duration which rotates disk 740 by 180degrees from the position in FIG. 11 to the position in FIG. 13. Att_(x), the microcontroller can generate a second pulse having a durationwhich rotates disk 740 by 180 degrees from the position in FIG. 13 tothe position in FIG. 11. Thus, the motor drive signal can be made up ofthese two pulses that are represented by dashed lines 764 whichrepresent the trailing edges of the pulses, as shown in motor drive plot420 of FIG. 4. As part of this embodiment, the flow diagram of FIG. 5may be modified such that step 516 drives motor 200 to move pivot point743 to its lowermost position so that the emergency collective actuatoris able to lower the collective in an engaged mode. Step 520, on theother hand, rotates the motor to position pivot point 743 at itsuppermost position to disengage the emergency collective actuator in adisengaged mode. Step 522 likewise drives the motor to position pivotpoint 743 at its uppermost position to disengage the emergencycollective actuator, if necessary.

FIG. 14 diagrammatically illustrates the appearance of a relevantportion of emergency collective actuator 10 in the case where the pilotis moving the collective upward, thereby counteracting the emergencycollective actuator which has previously lowered the collective, asevidenced by pivot point 743 being located in its lowermost position.The actuation by the pilot causes actuator spring 748 to extend suchthat ends 725 of lever arms 720 a and 720 b rotate in a clockwisedirection. It is noted that the length of the collective arm providesthe pilot with significant leverage for purposes of causing theextension of the actuator spring. It is considered that theconfiguration that has been shown by way of example may be modified byone having ordinary skill in the art in a wide variety of ways whileremaining within the scope of these teachings.

FIG. 15 is a block diagram illustrating another embodiment of anemergency collective actuator that is generally indicated by thereference number 800. Embodiment 800 shares many of the componentsdescribed above with reference to embodiment 300 of FIG. 3. In thisembodiment, however, timer 310, AND gate 312 and driver 314 receiveelectrical power from a power controller 802 via a power line 804. Inone embodiment, power controller 802 can be an electrical switch suchas, for example, a toggle switch that is mounted for actuation by thepilot. The pilot can therefore selectively use the switch to disable theemergency collective actuator, for example, when performing low altitudemaneuvering or hovering at which there is insufficient altitude forpurposes of autorotation. In such an instance, as discussed above, thepilot should react by increasing the collective and allow the helicopterto settle to the ground, using inertia that is present in the rotorsystem.

Turning to FIG. 16 in conjunction with FIG. 15, the former is adiagrammatic plan view of a helicopter 900 using another embodiment ofthe emergency collective actuator in which power controller 802 is usedwith a ground proximity detection unit 910. In one embodiment, theground proximity detection unit transmits a radar signal 912 that isused to detect the immediate distance to a surface 914 of the groundbased on a reflected signal 920. Controller 802 can be configured toautomatically disconnect output power from timer 310, AND gate 312 anddriver 314, and/or one or any combination of these components, belowsome predetermined altitude such as, for example, twenty feet.Controller 802 can receive a signal from the ground proximity detectionunit on an altitude input 930 (shown as a dashed line). In anotherembodiment, controller 802 can additionally be configured with anairspeed input 932 (shown as a dashed line) to receive the airspeed froman airspeed sensor 934 on the helicopter so as to use both altitude andairspeed to determine an appropriate combination of minimum altitude andminimum velocity below either of which the emergency collective actuatoris automatically disabled. It should be appreciated that sufficientforward speed would increase the appropriate low altitude to some degreeby providing the capability to contribute inertia to the rotor system.In one implementation, lookup tables based on combinations of airspeedand altitude can be formulated and stored in controller 802 based on theheight-velocity diagram for a given helicopter in which the system is tobe installed. Power controller 802 can then operate in accordance withaltitude 930 and airspeed 932 inputs based on the lookup table(s). Itshould be appreciated that helicopter manufacturers routinely generateheight-velocity diagrams for their helicopters. Such diagrams illustrateregions of safe and unsafe operation. Generally, as altitude increases,forward airspeed becomes relatively less critical. In one embodiment,the emergency collective actuator can be disabled from lowering thecollective when detected altitude and airspeed indicate that thehelicopter is operating within one or more predetermined unsafe regionsof the height-velocity diagram for that helicopter. Generally, apredetermined unsafe region can be considered to include the border ofthat region although an additional safety margin could be included whichwould slightly expand one or more of the predetermined unsafe regions.In any embodiment for which controller 802 provides for automaticoperation, input power for the controller can be provided via a switchthat is inserted in a series connection in power line 804 such that thepilot can manually actuate the switch to disable or enable automaticoperation of controller 802.

Attention is now directed to FIGS. 17 and 18 which illustrate yetanother embodiment of emergency collective actuator 100 in a partialperspective views. Main unit 110 includes a frame 1000 that can beformed, for example, from aluminum. The frame is mounted, for example,on floor panel 12 and supports motor 200. As seen in FIG. 18, frame 1000also supports control section 300. A toggle switch 1004, by way ofnon-limiting example, can be provided to allow the pilot toenable/disable the main unit. A clamp arrangement 1010 is attached tointermediate position 116 on the collective and engages one end of aflexible cable 1014. At least a portion of the cable proximate to anopposite end 1016 is received around a cable drum 1020. In the presentembodiment, the opposite end of cable 1014 receives a ball end (onlypartially visible in FIG. 18) that can be fixedly attached to the cableend, for example, by swaging. FIGS. 17 and 18 correspond to thecollective in a fully lowered position which results in a minimumcollective pitch setting that is available for the rotor system of thehelicopter.

FIGS. 19 and 20 are partial perspective views that correspond at leastgenerally to the perspective views of FIGS. 17 and 18, respectively, butwhich show collectively 20 in a fully raised, maximum collective pitchsetting for the rotor system of the helicopter. Thus, a relativelylonger length of cable 1014 is unwrapped from cable drum 1020 in FIGS.19 and 20 as compared to FIGS. 17 and 18. It is noted that collectivehandle 20 has been rendered as transparent in the further enlarged viewsof FIGS. 18 and 20 due to illustrative constraints. Further details willbe provided immediately hereinafter with regard to the operation of thecable drum and associated components.

Referring to FIG. 21, a perspective view is provided including motor200, as seen in association with an exploded, perspective view of anembodiment of a clutch arrangement 1022 which itself supports anembodiment of cable drum 1020. A stop bracket 1028 including a stopprong 1030 can be mounted to frame 1000 (also see FIGS. 17-20), forexample, using a pair of threaded fasteners. Motor 200 includes a motorshaft 1032. Initial descriptions will focus on components that alwaysco-rotate with the motor shaft. Accordingly, an intermediate shaft 1034can include a split end defining a complementary aperture (not shown)for receiving the motor shaft and an outer conical surface having aconical portion to provide for clamping engagement of the split endagainst the motor shaft, as will be described. An outer end ofintermediate shaft 1034 can define a threaded opening 1040 that isconfigured to receive a bolt 1042. In the final assembly, a clutch shaft1050 including a clutch disk 1052 is received on intermediate shaft 1034and includes an interior surface that is complementary to the outerconical surface of the intermediate shaft. Bolt 1042, when tightened,biases clutch shaft 1050 against the conical surface of the intermediateshaft such that the split end of the intermediate shaft clamps againstthe motor shaft.

Still referring to FIG. 21, the clutch arrangement includes a pair offriction rings 1060 a and 1060 b that are received against opposingshoulders 1054 (only one of which is visible) defined by clutch disk1052. A clutch drum 1064 defines an interior groove (not shown) thatreceives friction ring 1060 b. On the opposite side of the frictionplate, a Belleville washer 1068 is biased against friction ring 1060 aby a clutch housing plate 1070. The latter can be secured to clutch drum1064 by a plurality of fasteners that extend through openings in theclutch housing plate and into threaded openings 1074 in the clutch drum.Thus the clutch drum assembled to the clutch plate uses Bellevillewasher 1070 to resiliently capture the friction rings against the clutchdisk with the clutch drum and clutch plate supported for relativerotation around clutch disk 1052. A dowel 1080 is received in an opening1082 that is defined by clutch drum 1064. A stop pin 1084 can serve asone of a number of fasteners for securing the clutch plate to the clutchdrum and additionally includes a head for engaging against the sidemargins of stop prong 1030. It is noted that the remaining fasteners(not shown) for securing the clutch plate are of a lower profile and donot include a head that is configured to engage the stop prong. As willbe further described, the clutch arrangement provides a breakaway forcethat allows motor 200 to readily retract the collective, but alsoreadily allows the pilot to resist the actuation force from the motor bycausing the friction rings of the clutch arrangement to slip againstclutch disk 1052. A snap ring 1086 can hold cable drum 1020 on clutchshaft 1050. It is noted that the components of the clutch arrangement ofFIG. 21 can be formed from any suitable material including but notlimited to aluminum and stainless steel, as will be readily apparent tothose of ordinary skill in the art.

Referring to FIGS. 22 and 23 in conjunction with FIG. 21, a pair ofbearings 1090 can be received on clutch shaft 1050 for rotationallysupporting cable drum 1020. The cable drum receives bearings 1090 withina cavity 1092 of the cable drum which is seen in FIG. 22, showing anembodiment of the interior configuration of the cable drum. A collar1094 can be integrally formed with clutch drum 1064 and projectsoutwardly therefrom. The collar can define a pair of opposing slots1096. A constant force spring 1100 includes an inside diameter that isreceivable on collar 1094 and includes an inside end tab 1102 that isreceivable in one of slots 1096. An opposing, outer end of spring 1100can include a rolled end 1104. Cable drum 1020, when supported bybearings 1090, interiorly receives spring 1100 having rolled end 1104received in a groove 1110 (FIG. 22) that can be cylindrical inconfiguration. A dashed circle 1110′ (FIG. 23) is a projection of thecylindrical groove onto the end surface of the cable drum to show itsrelative position. Thus, rotation of the cable drum in acounterclockwise direction relative to clutch drum 1064, in the view ofFIG. 21, results in compressing or winding constant force spring 1100.As seen in FIG. 23, cable 1014 is partially wound around the cable drumand a suitable fitting such as, for example, a ball 1120 can be fixedlyattached to each end of the cable. As seen in FIG. 21, cable drum 1020can define a passage 1122 having an enlarged end opening for initiallyreceiving ball 1120. The ball is then moved outward into the length ofthe passage to a captured position, as illustrated by FIG. 23. As seenin FIG. 22 and as will be further described, cable drum 1020 defines achannel 1130 that is at least generally circular and which definesopposing first and second channel ends 1132 a and 1132 b, respectively.When the cable drum is assembled against clutch drum 1064, an outwardend of dowel 1080 is received in channel 1130 such that channel ends1132 a and 1132 b serve as end stops with respect to the travel of thedowel in the channel.

Attention is now directed to FIGS. 24 a-24 d which are diagrammatic endview illustrations, based on the emergency collective actuatorembodiment of FIGS. 19-23, that are limited to showing the relationshipbetween stop prong 1030 of stop bracket 1028, stop pin 1084 and dowel1080 in different operational states. Initially, in FIG. 23, theemergency collective actuator is shown with the collective in the fullup position of FIGS. 19 and 20 having the emergency collective actuatorin an inactivate or neutral state such that there is no discernableinfluence on the pilot's choice of collective setting. The emergencycollective actuator, however, is ready to lower the collective in theevent that a low RPM signal is received. In this state, cable 1014 (seeFIGS. 19 and 20) is unwound to a maximum extent from cable drum 1020through clockwise rotation which winds constant force spring toexperience its maximum operational tension. At the same time, dowel 1080is positioned against or nearly against first end 1132 a of channel 1130while stop pin 1084 is engaged against a first side of stop prong 1030.It should be appreciated that the position of stop pin 1084 isindicative of the operational status of the emergency collectiveactuator. In FIGS. 24 a, 24 b and 24 c, the stop pin being biasedagainst the first side of the stop prong indicates that the emergencycollective actuator is in the inactive or neutral state. If the stop pinis engaged against a second, opposite side of the stop prong as seen inFIG. 24 d, the emergency collective actuator is in an actuated state andhas at least attempted to pull down the collective to the minimumposition shown in FIGS. 17 and 18.

Referring to FIG. 24 b and as noted above, the emergency collectiveactuator is shown in its inactive state but remains ready to pull downthe collective as is evidenced by the position of stop pin 1084. Thecollective has been lowered to some extent by the pilot, as compared toFIG. 24 a, such that constant force spring 1100 (FIG. 21) rotates cabledrum 1020 counterclockwise, as indicated by an arrow 1200, to take upslack in the cable. At the same time, channel 1130 rotates with thecable drum such that dowel 1080 is moved away from first end 1132 a ofthe channel and toward second end 1132 b. If the pilot continues tolower the collective to its minimum position, dowel 1080 will bereceived against or nearly against second end 1132 b of the channel asillustrated by FIG. 24 c.

FIG. 24 d diagrammatically illustrates the emergency collective actuatorafter having been triggered by a low RPM signal to rotate the clutcharrangement counterclockwise in the direction of arrow 1200 such thatstop pin 1084 is received against a second, opposite side of stop prong1030. At some point during the clutch rotation, which depends on thecurrent pilot selected position of the collective control, dowel 1080can engage first end 1132 a of groove 1130 to then rotate the clutchdrum in a way that winds cable 1014 (see FIGS. 18 and 23) onto the cabledrum and pulls down the collective. Even if the collective is alreadyfully lowered, the mutual relationships between the various componentswill be achieved as illustrated by FIG. 24 d. Once the collective ispulled down or at any time during the actuation that is intended to pulldown the collective, the pilot can pull the collective up to overcomethe actuation force which can cause the clutch arrangement to slip,thereby allowing rotation of cable drum 1020 in a clockwise direction,opposite arrow 1200.

The clutch arrangement of FIGS. 17-24, utilizes resilient biasing toeliminate cable slack while allowing the pilot to operate the collectiveduring normal operational conditions. When the emergency collectiveactuator is actuated, the pilot is able to overcome the actuation forcevia clutch slippage. In some embodiments, a rigid linkage can be usedwithout the need for a flexible cable. Accordingly, the rigid linkagecan be pivotally connected, for example, to clutch drum 1064.

FIG. 25 illustrates another embodiment of emergency actuator 10 whichcan be understood based on the descriptions of FIG. 2 appearing above.For this reason, the present descriptions are limited to describing thevarious ways in which the present embodiment differs from the embodimentof FIG. 2. In particular, a modified cable drum 204′ has been providedthat is not cylindrical, but rather is frustoconical in form. Cable 112passes around an idler 1300 and is received in a spiral groove 1302. Oneend of the cable can be fixed within groove 1302, for example, by afastener 1303. When motor 200 rotates drum 204′ in a direction indicatedby an arrow 1304, cable 112 is taken up in groove 1302 to retract orlower collective 20. The present embodiment is different from theembodiment of FIG. 2, however, at least to the extent that as theretraction progresses, the rate of retraction decreases. It should beappreciated that the specific shape of the spiral groove in the sidewallcan be configured to cooperate with the shape of the sidewall of thedrum itself to provide a wide range of customized retraction rateprofiles and is not limited to a continuous decrease in the rate ofretraction. That is, the retraction rate can decrease and/or increase inany suitable manner as the retraction progresses. In an embodiment, thedrum and spiral groove can be configured to provide for retraction inless than one full revolution of the drum. In another embodiment, theretraction rate can be customized by using the embodiments of FIG. 2 or17 while providing a variable drive signal to motor 200. For example,motor drive rate profiles 1310 can be stored in electronic controlsection 300. In this case, motor 200 can be a geared servo motor orother suitable type of motor.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. For example, in anembodiment, the low RPM signal can be generated based on the engine RPM.As shown in FIG. 3, a comparator 1400 (shown in phantom using dashedlines) can monitor the engine RPM based on a predetermined engine RPMthreshold to generate the low RPM signal at the output of the comparatorthat is used in FIG. 4. The RPM engine threshold can be customized inview of a given helicopter. In this way, at least some degree of evenfaster system response can be achieved. It is therefore intended thatthe following appended claims and claims hereafter introduced areinterpreted to include all such modifications, permutations, additionsand sub-combinations as are within their true spirit and scope.

What is claimed is:
 1. An apparatus for use with a helicopter includinga rotor system having a main rotor with an adjustable pitch that iscontrolled at least in part by a pilot using a collective control, anapparatus comprising: an actuator arrangement that is configured tochange the adjustable pitch by exerting an actuation force, whichactuation force can be overcome by the pilot using the collectivecontrol, and which actuation force otherwise biases the adjustable pitchtoward a minimum collective pitch position; and a control arrangementthat is configured to receive a signal that is indicative of a low RPMcondition of the main rotor and to respond to said signal during apredetermined period of time by activating the actuator arrangement toexert the actuation force.
 2. The apparatus of claim 1 wherein saidcontrol arrangement is further configured to periodically monitor thelow RPM signal during the predetermined period of time to detect achange in an active/inactive state of the low RPM signal.
 3. Theapparatus of claim 2 wherein the actuator arrangement includes a motorto apply the actuation force and the control arrangement, during thepredetermined period of time, is configured to turn the motor on if themotor is off and to maintain the motor in an on state if the motor isalready on responsive to detecting an active state of the low RPMsignal.
 4. The apparatus of claim 3 wherein the control arrangement isconfigured to turn off the motor responsive to detecting an inactivestate of the low RPM signal during the predetermined period of time. 5.The apparatus of claim 1 wherein the control arrangement is configuredto monitor the low RPM signal in a continuous loop and to initiate thepredetermined period of time responsive to detecting an active state ofthe low RPM signal.
 6. The apparatus of claim 5 wherein the controlarrangement, responsive to detecting the active state of the low RPMsignal, is further configured to thereafter reconfirm the active statebefore causing the actuator arrangement to exert the actuation force. 7.The apparatus of claim 1 wherein said control arrangement is furtherconfigured for entering a lockout interval immediately following saidpredetermined period of time during which lockout interval the Low RPMsignal is disabled from activating the actuator arrangement.
 8. Theapparatus of claim 1 wherein said predetermined period of time is in arange from 1 second to 10 seconds.
 9. The apparatus of claim 1configured to apply said actuation force within 1 millisecond ofreceiving the Low RPM signal.
 10. The apparatus of claim 1 wherein saidcollective control includes an arm that is pivotable to change theadjustable pitch of the rotor and said arm includes an arm lengthextending from a pivot end to a distal end that is directly engaged bythe pilot and wherein said actuator arrangement is connected to anintermediate position along the arm length to apply said actuation forcedirectly to the arm.
 11. The apparatus of claim 10 positioned below saidcollective control such that said force pulls on said collective controlfrom said intermediate position.
 12. The apparatus of claim 11 whereinsaid actuator arrangement includes a cable having one end that isattached to the intermediate position of the arm length for pulling onthe collective control.
 13. The apparatus of claim 12 wherein theactuator arrangement includes an electric motor for applying said forceto the cable by rotating in a given direction.
 14. The apparatus ofclaim 1 wherein said collective control includes an arm that ispivotable to change the adjustable pitch of the rotor and acounterbalance spring that is configured as part of a counterbalancelinkage to apply a resilient bias to the collective arm and wherein saidactuator arrangement is configured to apply said force to saidcounterbalance linkage in a way that compresses the counterbalancespring for moving the collective control to the minimum pitch position.15. The apparatus of claim 14 wherein said actuator arrangement furtherincludes at least one lever arm having a first end that is pivotallyattached to the helicopter and a second, opposing end that is movedresponsive to an electric motor such that an intermediate portion of thelever arm applies said force.
 16. The apparatus of claim 1 wherein saidcontrol arrangement includes a pilot actuatable switch that isswitchable by the pilot to selectively disable the application of saidforce to the collective.
 17. The apparatus of claim 1 wherein saidcontrol arrangement includes an arrangement for using at least adetected altitude of the helicopter above a surface of the ground todisable the application of said force below a predetermined altitude.18. The apparatus of claim 1 wherein said control arrangement isconfigured for disabling the application of said force based on adetected altitude of the helicopter in combination with a detectedairspeed of the helicopter.
 19. The apparatus of claim 1 wherein saidcontrol arrangement is configured for disabling the application of saidforce within at least one predetermined unsafe region of aheight-velocity diagram for the helicopter.
 20. A method for use with ahelicopter including a rotor system having a main rotor with anadjustable pitch that is controlled at least in part by a pilot using acollective control and which helicopter generates a Low RPM signal thatis indicative of a low RPM condition of the main rotor, said methodcomprising: monitoring the low RPM signal to initiate a predeterminedperiod of time; and during the predetermined period of time, changingthe adjustable pitch by selectively exerting an actuation forceresponsive to the low RPM signal, which actuation force can be overcomeby the pilot using the collective control, and which actuation forceotherwise biases the adjustable pitch toward a minimum collective pitchposition